Drilling assembly with underreaming bit and method of use

ABSTRACT

Embodiments provide apparatuses and methods for drilling and underreaming, particularly to underreamer expansion bits. Underreamer drill assemblies and underreaming bits manufactured in accordance with various embodiments may help to resist over-excavating or undermining, which may result in a more stable bore hole.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims priority to U.S. Patent Application No.61/298,490, filed Jan. 26, 2010, entitled “DRILL ASSEMBLY WITHUNDERREAMING BIT AND METHOD OF USE,” the entire disclosure of which ishereby incorporated by reference in its entirety.

TECHNICAL FIELD

Embodiments herein relate to the field of excavation, and, morespecifically, to apparatus and methods for drilling and underreaming.

BACKGROUND

Underreaming is an excavation technique used in boring and in installingpiles or steel casings. For instance, underreaming may be used toenlarge or ream a borehole beneath a string of casing or drivepipe.Expansion bits are useful for underreaming, but when different strataare encountered, particularly unconsolidated formations such as sand,gravel, clay, and water, conventional expansion underreamers can causeundermining or over-excavating of loose materials. This can underminethe stability of the hole.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be readily understood by the following detaileddescription in conjunction with the accompanying drawings. Embodimentsare illustrated by way of example and not by way of limitation in thefigures of the accompanying drawings.

FIG. 1 illustrates a side view of an underreamer assembly with arms inthe extended position, in accordance with various embodiments;

FIG. 2 illustrates a side view of an underreamer assembly with arms inthe retracted position, in accordance with various embodiments;

FIG. 3 illustrates an exploded view of an underreamer assembly, inaccordance with various embodiments;

FIG. 4 illustrates a side view of a center shaft, in accordance withvarious embodiments;

FIG. 5 illustrates a cross-sectional view of a center shaft, inaccordance with various embodiments;

FIG. 6 illustrates a perspective view of a center shaft, in accordancewith various embodiments;

FIGS. 7A, 7B, and 7C illustrate a side view of a bit body (FIG. 7A), aside view of a spring retainer (FIG. 7B) and cross sectional view of abit body (FIG. 7C), in accordance with various embodiments;

FIG. 8 illustrates a perspective view of a bit body, in accordance withvarious embodiments;

FIGS. 9A, 9B, and 9C illustrate a perspective view (FIG. 9A), side view(FIG. 9B), and back view (FIG. 9C) of an arm, in accordance with variousembodiments;

FIG. 10 illustrates a cross-sectional view of a center shaft with theair flow pathway indicated, in accordance with various embodiments;

FIG. 11 illustrates a face view of the underreamer assembly in theextended position, in accordance with various embodiments;

FIG. 12 illustrates a perspective view of the assembly of variousinterchangeable components, in accordance with various embodiments;

FIG. 13 illustrates a perspective view of the flow of flush mediathrough an exemplary underreamer assembly, in accordance with variousembodiments;

FIG. 14 illustrates a cross-sectional view of the flow of flush mediathrough an exemplary underreamer assembly, in accordance with variousembodiments;

FIG. 15 illustrates a side view of an alternate embodiment of theunderreamer assembly configured to be driven by a hex drive mechanism,in accordance with various embodiments;

FIGS. 16A, 16B, 16C, and 16D illustrate a side view of an embodiments ofa bit body (FIG. 16A), a side view of a spring retainer (FIG. 16B), across sectional view of a bit body (FIG. 16C), and a transverse crosssectional view of a bit assembly (FIG. 16D), all components of a bitassembly configured for use with a hex drive mechanism, in accordancewith various embodiments;

FIG. 17 illustrates a side view of another alternate embodiment of theunderreamer assembly configured to be driven by a hex drive mechanism,in accordance with various embodiments; and

FIG. 18 illustrates a cross sectional view of components of a bitassembly configured for use with a hex drive mechanism, in accordancewith various embodiments.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which are shownby way of illustration embodiments that may be practiced. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope. Therefore,the following detailed description is not to be taken in a limitingsense, and the scope of embodiments is defined by the appended claimsand their equivalents.

Various operations may be described as multiple discrete operations inturn, in a manner that may be helpful in understanding embodiments;however, the order of description should not be construed to imply thatthese operations are order dependent.

The description may use perspective-based descriptions such as up/down,back/front, and top/bottom. Such descriptions are merely used tofacilitate the discussion and are not intended to restrict theapplication of disclosed embodiments.

The terms “coupled” and “connected,” along with their derivatives, maybe used. It should be understood that these terms are not intended assynonyms for each other. Rather, in particular embodiments, “connected”may be used to indicate that two or more elements are in direct physicalor electrical contact with each other. “Coupled” may mean that two ormore elements are in direct physical or electrical contact. However,“coupled” may also mean that two or more elements are not in directcontact with each other, but yet still cooperate or interact with eachother.

For the purposes of the description, a phrase in the form “A/B” or inthe form “A and/or B” means (A), (B), or (A and B). For the purposes ofthe description, a phrase in the form “at least one of A, B, and C”means (A), (B), (C), (A and B), (A and C), (B and C), or (A, B and C).For the purposes of the description, a phrase in the form “(A)B” means(B) or (AB) that is, A is an optional element.

The description may use the terms “embodiment” or “embodiments,” whichmay each refer to one or more of the same or different embodiments.Furthermore, the terms “comprising,” “including,” “having,” and thelike, as used with respect to embodiments, are synonymous.

In various embodiments, methods, apparatuses, and systems forunderreaming are provided. Embodiments herein provide underreamingdevices, such as bits and bit assemblies, and methods for drilling andunderreaming that overcome the shortcomings of conventional underreamingdevices. Among other things, underreamers may be useful for a variety ofexcavation tasks, such as expanding well bores, for instance to increasethe yield of the well; for straightening a bend in a hole, which mayprevent the advancement of a pile or casing; and/or for excavatingtie-back or anchor holes in any type of dirt, rock, or concreteformations.

Underreamers typically have cutting members that are designed to bemoved or extended against a well bore wall after the tool is positionedwithin the well bore. However, such underreamers typically have an airor fluid media flow pattern that forces air or fluid media to exit thebit assembly in a lateral direction, e.g., against the sides of thehole. This may be problematic when excavating different strata,particularly unconsolidated formations such as sand, gravel, clay, andwater, as the pressure and turbulence of the media can undesirablyenlarge the hole in an uneven fashion depending on the solidity of thestrata. These unconsolidated soil conditions may occur deep in the hole,for instance, beneath rock strata, and conventional expansionunderreamers may therefore cause undermining or over-excavating of theloose materials without the knowledge of the operator. This mayundermine the stability of the hole and/or any casing, footing, or pilecontained therein. By contrast, the underreamers provided herein invarious embodiments may provide a substantially vertical downward mediaflow pattern through the bottom of the bit, which preventsover-excavation from lateral flow and/or turbulence, and which mayincrease the stability of the resulting hole.

Another advantage of the disclosed underreamer devices is that it mayhave a short vertical distance between the pilot area of the cuttingface (e.g., the distal-most portion of the bit) and the proximal edgesof the underreaming arms. On a conventional underreaming bit, thisdistance is about 8-10 inches, requiring the pilot area of the bit toadvance that distance in front of the underreaming zone. On thedisclosed underreaming bits, this distance may be much shorter, forexample, about four, three, or two inches, or even less. In variousembodiments, reducing this distance may reduce the mass of the bit,which increases the energy transformation into the drill, making for amore effective boring mechanism.

In various embodiments, the shorter working distance between the pilotarea of the cutting face and the underreaming arms also may increase theefficacy of the underreaming action. When a pile is being driven intothe ground and becomes obstructed by a boulder, an underreamer may beused to remove the obstruction and allow the pile to continue toadvance. However, if there is a long working distance between the pilotarea and the underreaming arms, it may not be possible to advance theunderreaming assembly far enough for the arms to be effective atclearing the obstruction. Thus, the short working distance between thepilot area and the underreaming arms in the bits of the presentdisclosure may allow the disclosed underreamer devices to be used insuch a situation.

In other embodiments, a further advantage of the disclosed underreamingbits and assemblies is that many of the components are replaceableand/or interchangeable. Thus, when components of the assembly becomeworn, they may be replaced with new components without having to replacethe whole bit or bit assembly. For instance, the buttons on the arms orpilot face may be replaced, as may the outer body, and the individualarms. Furthermore, in some embodiments, the center shaft may beexchanged with a different center shaft, and/or a different outer bodymay be substituted, allowing the device to be configured to suit thespecific conditions of the task at hand.

Additionally, the design of the bit assembly and center shaft may resultin a highly efficient torque transfer through the center shaft. This mayincrease the efficiency of the system, and may increase the boring andunderreaming efficacy of the device as compared to conventional devices.

FIG. 1 shows an exemplary underreaming device 10 with the arms 70 in anextended position, and FIG. 2 illustrates the embodiment shown in FIG. 1with the arms 70 in a retracted position. FIG. 3 illustrates an explodedview of the underreamer assembly shown in FIG. 1, and FIG. 4 illustratesa side view of the exemplary center shaft shown in FIG. 1, in accordancewith various embodiments. FIG. 5 illustrates a cross-sectional view of acenter shaft, in accordance with various embodiments. FIG. 6 illustratesa perspective view of a center shaft, in accordance with variousembodiments.

Referring to FIG. 1, in general, the underreaming device 10 may includea drive device coupler 20, which, in some embodiments, may couple theunderreaming device to a drive device, and which may form a part of andcommunicate rotational torque, impact, vibration and/or linear force tothe center actuating shaft 30, which may in turn transfer rotationaltorque, impact, and/or vibration to the bit body 50 and/or to one ormore arms 70. In some embodiments, such as the illustrated embodiment,two arms 70 are used. However, any number of arms may be used, dependingon the particular excavation conditions, the size of the arms, and thediameter of the bit body 50 being used. For instance, in variousembodiments, underreaming device 10 may have one, two, three, four,five, six, seven, or even more arms, depending on the intended use, thediameter of bit body 50, the diameter of the hole being excavated, orthe substrate in which underreaming device 10 is used. In someembodiments, bit body 50 may be configured to have a size appropriatefor passing inside a standard steel pipe or casing.

In various embodiments, bit body 50 may transfer torque to and/or serveas a guide for arms 70. In use, in various embodiments, arms 70 maytravel from the extended position shown in FIG. 1 to the retractedposition illustrated in FIG. 2 and back again repeatedly in order toeffect an underreaming function. FIG. 3 illustrates the relationshipbetween arms 70 and bit body 50, and in particular, how they rest inapertures formed by perpendicular surfaces 60 and primary arm pocketsurfaces 61 in bit body 50. In some embodiments, bit body 50 may rotatein a clockwise or counterclockwise direction (or alternate directions)during use, and may advance in an axial (e.g., distal) direction as thehole is excavated.

Referring to FIG. 4, which shows center actuating shaft 30 in side view,and FIG. 5, which shows center actuating shaft 30 in cross section,center actuating shaft 30 may have a central hole 22 for air, water, orany other flush media to pass though. Center actuating shaft 30 may alsohave a main section 32 having a specified diameter, and an adjacent(e.g., approximately 90 degree) face that fits within bit body 50,according to various embodiments. Center actuating shaft 30 may, in someembodiments, guide bit body 50 and transfer impact to the adjacent face(e.g., top or proximal edge) of bit body 50. Center actuating shaft 30also may have an undercut section 34 having a specified diameter withtwo adjacent (e.g., approximately 90 degree) faces. In one embodiment,undercut section 34 may have a specified diameter, and may have a sizesufficient to allow a spring retainer (not shown) to be collapsed forassembly and disassembly, and to be positioned to keep arm 70 engaged inthe retracted position. Although in the illustrated embodiment, thespring retainer functions to couple bit body 50 to center actuatingshaft 30, any other retention device may be used for this purpose, aswill be appreciated by those of skill in the art.

In various embodiments, center actuating shaft 30 also may have airholes 36 that communicate with the main central hole 22. Air holes 36may be used for cleaning debris from the mechanism, according to variousembodiments. Embodiments of center actuating shaft 30 also may have anextended step surface 38 with adjacent sides parallel to centeractuating shaft 30. A ramp angle surface 40 may serve as a transferbetween the extended step surface 38 and a retract step surface 42,which may have adjacent sides that are substantially parallel to theshaft. According to various embodiments, ramp angle surface 40 may haveone or more air holes 44 that communicate with central hole 22. Inembodiments, air holes 44 may be used for clearing debris or other mediafrom the bit assembly. In addition, center actuation shaft 30 may have aradial air groove 46. Some embodiments may provide a face 48perpendicular to center actuating shaft 30, and in some embodiments face48 may include one or more carbide (or other rock cutting material)compact buttons or inserts 90.

FIGS. 7A, 7B, and 7C illustrate a side view of a bit body (FIG. 7A), aside view of a spring retainer (FIG. 7B) and cross sectional view of abit body (FIG. 7C), in accordance with various embodiments. Referring toFIGS. 7A-7C, bit body 50 may have a main diameter bore 52 with anadjacent face that may guide bit body 50 on center shaft 30 and maytransfer impact force through the adjacent face to the face of centeractuating shaft 30. Also included in some embodiments is a radial groove54 that has two adjacent faces that fit a spring retainer and/or springretainer ring 55. In some embodiments, holes 56 run perpendicularlythrough bit body 50 and center actuation shaft 30. In use, keys, pins,or other elongated objects may be inserted through holes 56 to collapsespring retainer ring 55 and uncouple bit body 50 from center actuatingshaft 30. Thus, bit body 50 may be easily uncoupled from centeractuating shaft 30, for instance when bit body 50 requires replacing dueto wear, when a different diameter bit body 50 is desired, or when arms70 need to be accessed or replaced.

In various embodiments is a radial pocket surface 58 that may act as astop to prevent over extension of the arms 70 in the extended position.In some embodiments, perpendicular surfaces 60 are included and disposedto mate with arms 70. Additional surfaces may include primary arm pocketsurfaces 61 and secondary arm surfaces 62, both of which may begenerally perpendicular to center actuation shaft 30. Some embodimentsmay also include a radial angle surface 63 that may include compactinserts 90. Additionally, bit body 50 may include one or more faceflushing slots 64 running perpendicular to center actuating shaft 30,and main return flushing slots 66 parallel to center actuating shaft 30that may intersect with one or more face flushing slots 64. Someembodiments also may include a face 68 generally perpendicular to centeractuating shaft 30 that may have additional carbide (or other cuttingmaterial) compact buttons or inserts 90 coupled thereto.

FIG. 8 illustrates a perspective view of an example of bit body 50, inaccordance with various embodiments, and FIGS. 9A, 9B, and 9C illustratea perspective view (FIG. 9A), side view (FIG. 9B), and back view (FIG.9C) of an example of arm 70, in accordance with various embodiments.Referring to FIG. 9, arm 70 may be a removable and replaceable componentthat has a radial diameter surface 72 that may define a ream diameter,and a radial plot surface 74 having a specified diameter. Also includedin some embodiments is a radial angle surface 76 that has inserts 90coupled thereto. A perpendicular face 78 also may have compact inserts90, and may act as part of the pilot surface in some embodiments.Another perpendicular surface 80 may be configured to contact and matewith the bit body arm pocket 60. Also included in some embodiments is aplurality of major arm surfaces 81 that may contact and mate with themajor arm pocket surfaces 61. Also contemplated are minor arm surfaces82 that may contact and mate with minor arm surfaces 62.

Some embodiments may include one or more radial stop surfaces 84, whichmay contact and mate with bit body radial pocket surface 58. Furtherembodiments also may include a radial back angle surface 86 for aidingin the retractions of radial arms 70. A parallel actuating surface 88may contact and mate with center actuating shaft 30 during operation,and in some embodiments it may contact and mate with retract stepsurface 42, the ramp angle surface 40, and/or extended step surface 38.Embedded in the surface may be buttons or compact inserts 90, accordingto various embodiments.

FIG. 10 illustrates a cross-sectional view of an example of centeractuating shaft 30 with the air or media flow pathway indicated, inaccordance with various embodiments, and FIG. 11 illustrates a face viewof the underreamer assembly 10 with arms 70 in the extended position, inaccordance with various embodiments.

In operation, in various embodiments, drive device coupler 20 (which insome embodiments may be an integral part of center actuating shaft 30)may be the adapter to which a down hole hammer, drill rod for hydraulichammer, or any other means to impart rotational torque, impact energy,vibration, or linear feed force may be used to move (for instance rotateor advance) center shaft 30. In various embodiments, center actuatingshaft 30 (and thus coupler 20) may be interchanged with othershafts/couplers in order to change the assembly to be, for instance,mated with a down hole hammer or a drill rod for a hydraulic hammer.

In some embodiments, the rotational torque from center actuating shaft30 may be imparted to arms 70 by means of contact with surfaces 42, 44,and 38. The rotational torque may then be transferred from arms 70 intobit body 50 in some embodiments by means of contact with surfaces 81 and82 of arms 70 and their mated surfaces 61 and 62 bit body 50.

FIG. 15 illustrates a side view of an alternate embodiment of theunderreamer assembly configured to be driven by a hex drive mechanism;and FIGS. 16A, 16B, 16C, and 16D illustrate a side view of anembodiments of a bit body (FIG. 16A), a side view of a spring retainer(FIG. 16B), a cross sectional view of a bit body (FIG. 16C), and atransverse cross sectional view of a bit assembly (FIG. 16D), allcomponents of a bit assembly configured for use with a hex drivemechanism, in accordance with various embodiments. In these embodiments,instead of imparting torque directly to arms 70, which then imparttorque to bit body 50, rotational torque from center actuating shaft 30may be imparted to bit body 50 via drive members 31, 51. Although a hexdrive interface is illustrated, any torque transfer interface may besubstituted, for instance a spline drive mechanism or any interfacehaving any number of flat surfaces, facets, recesses in, or projectionsfrom center shaft 30 that are configured to mate with and/or engagecorresponding features in bit body 50. In various embodiments, use ofthe hex/spline drive mechanism (for instance, drive members 31, 51) mayhelp to avoid excessive loading and wear on surfaces 38. 40, and 42 oncenter actuating shaft 30, and mated surfaces 61 and 62 on bit body 50,while maintaining alignment of the components and ensuring smoothoperation of arm 70 deployment and retraction. Thus, the illustrated hexdrive mechanism may help avoid causing excessive wear and tear on arms70 and bit body 50.

Whereas FIGS. 15 and 16 illustrate an example of a center shaft 30/bitbody 50 drive members 31, 51 being located proximal to spring retainer55, one of skill in the art will appreciate that such drive members 31,51 may alternately or additionally be located distal to spring retainer55, as shown in FIGS. 17 and 18. FIG. 17 illustrates a side view ofanother alternate embodiment of the underreamer assembly configured tobe driven by a hex drive mechanism, in accordance with variousembodiments; and FIG. 18 illustrates a cross sectional view ofcomponents of a bit assembly configured for use with a hex drivemechanism, in accordance with various embodiments. In some embodiments,such as the embodiment illustrated in FIGS. 17 and 18, a hex drive,spline drive, or other drive member 31, 51 may be located at any pointbetween the distal tip of center shaft 30 and spring retainer 55. Insome embodiments, positioning drive members 31, 51 distal to (e.g.,below) spring retainer 55 may increase the area of contact betweenopposing drive members 31, 51, which may reduce wear and may decreasethe height of bit body 50 in some embodiments.

In some embodiments, arms 70 may be deployed by the linear (e.g. axial)force applied to center actuating shaft 30 and transferred to bit body50. As bit body 50 moves, arms 70 may be engaged by means of contactwith surfaces 88, 42, 44, and 38. At the point of full deployment, arms70 may be kept from deploying further by means of surfaces 84 and 58,which collectively may be referred to as the “stop pocket.” Impactenergy may be imparted from center actuating shaft 30 by means ofsurface 32 and its adjacent surface 42 in bit body 50, and into arms 70by means of surfaces 60 and 80, in accordance with various embodiments.

FIG. 12 illustrates a perspective view of the assembly of variousinterchangeable components. As described above, one advantage of thedisclosed underreaming bits and assemblies is that many of thecomponents are replaceable and/or interchangeable. Thus, when componentsof the assembly become worn, they may be replaced with new componentswithout having to replace the whole bit or bit assembly. For instance,the buttons on the arms or pilot face may be replaced, as may bit body50, and individual arms 70. Furthermore, in some embodiments, centershaft 30 may be exchanged with a different center shaft, and/or adifferent bit body 50 may be substituted, allowing worn components to bereplaced, and/or the device to be configured to suit the specificconditions of the task at hand.

As described above, in use, retention device 55 may be collapsed orotherwise disengaged, for instance by inserting keys, pins, or otherelongated objects through holes 56, thus uncoupling bit body 50 fromcenter actuating shaft 30. Once bit body 50 has been removed from centeractuating shaft 30, arms 70 may be removed from the interior of bit body50 by retracting them into bit body 50 and lifting them out of therecesses in which they rest. Replacement arms may then be inserted intobit body 50 if desired, and/or a replacement bit body 50 may be coupledto center shaft 30, for instance one having a larger or smallerdiameter, a different number of arms, or other desirable features, suchas a different drive mechanism.

FIG. 13 illustrates a perspective view of the flow of air, water, and/orflush media through an exemplary underreamer assembly; and FIG. 14illustrates a cross-sectional view of the flow of air, water, and/orflush media through an exemplary underreamer assembly; all in accordancewith various embodiments. As illustrated in in FIGS. 13 and 14, invarious embodiments, substantially all of the air, water, and/or flushmedia may pass through the underreamer assembly and exit through thedistal cutting face in a downward direction, substantially parallel tothe longitudinal axis of the device and perpendicular to the distalcutting face. In various embodiments, such downward airflow may directsubstantially all of the air, water, and/or flush media away from theside (lateral) walls of the bore hole, thus preventing over excavationwhen loose strata are encountered during a boring operation. Inparticular embodiments, unlike conventional underreamers, essentially noflush media pass laterally against the sides of the bore hole, as anyair, water, and/or flush media traveling laterally towards the bore holewalls will be diverted by arms 70 and/or the outer edge of bit body 50,particularly when arms 70 are in the extended position. Thus, in variousembodiments, at least 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%,98% or 99% of the air, water, and/or flush media may be directed awayand along the central axis and perpendicular to the distal cutting face.Similarly, in some embodiments, less than 20%, 15%, 10%, 9%, 8%, 7%, 6%,5%, 4%, 3%, 2%, or 1% of the air, water, and/or flush media may bedirected laterally towards the side wall of the bore hole. Thus, thedevice may avoid the over-excavation problems associated withconventional underreamers.

Debris removal may be accomplished in some embodiments by flush media ofair, water, or any combination of fluid or gas, through central hole 22,into air holes 44, and deflected by surface 88 of arms 70. Followingthat, the flush media may move around center actuating shaft 30 andradial air groove 46 in some embodiments, e.g., to exit the assembly ina direction parallel to the drilled hole, without impacting the sides ofthe hole. The media may then be returned via face flushing slot 64 intoreturn flushing slot 66 in some embodiments. A small amount of flushmedia may be diverted to holes 36 to keep the mechanism clean of debrisin certain embodiments.

Although certain embodiments have been illustrated and described herein,it will be appreciated by those of ordinary skill in the art that a widevariety of alternate and/or equivalent embodiments or implementationscalculated to achieve the same purposes may be substituted for theembodiments shown and described without departing from the scope. Thosewith skill in the art will readily appreciate that embodiments may beimplemented in a very wide variety of ways. This application is intendedto cover any adaptations or variations of the embodiments discussedherein. Therefore, it is manifestly intended that embodiments be limitedonly by the claims and the equivalents thereof.

1. An underreamer drill assembly comprising: a center shaft having acentral axis, a proximal end, and a distal end, wherein the center shaftis configured to receive drive energy from a driver at the proximal endand adapted to couple to a bit body at the distal end; a bit bodyadapted to couple to the center shaft and configured to couple to atleast one underreamer arm; and at least one underreamer arm configuredto underream a bore hole having a side wall, wherein the center shaft,bit body, and underreamer arm form a distal cutting face that issubstantially perpendicular to the central axis, and wherein theunderreamer drill assembly is configured to direct air, water, or flushmedia substantially away from the side wall of the bore hole.
 2. Theunderreamer drill assembly of claim 1, wherein the center shaft and bitbody are adapted to direct air, water, or flush media substantiallyalong the central axis and perpendicular to the distal cutting face. 3.The underreamer drill assembly of claim 1, wherein the center shaft andbit body are adapted to direct air, water, and flush media substantiallyalong the central axis and perpendicular to the distal cutting face. 4.The underreamer drill assembly of claim 1, wherein the at least oneunderreamer arm is configured to be extended against the side wall ofthe bore hole.
 5. The underreamer drill assembly of claim 1, wherein theat least one underreamer arm comprises a distal pilot surface, aproximal upper surface, and an axial height spanning the distancebetween the distal pilot surface and the proximal upper surface.
 6. Theunderreamer drill assembly of claim 5, wherein the axial height is lessthan 8 inches.
 7. The underreamer drill assembly of claim 5, wherein theaxial height is less than 6 inches.
 8. The underreamer drill assembly ofclaim 5, wherein the axial height is between about 2 and about 4 inches.9. The underreamer drill assembly of claim 1, wherein the distal cuttingface comprises a plurality of compact cutting inserts.
 10. Theunderreamer drill assembly of claim 9, wherein the compact cuttinginserts are replaceable.
 11. The underreamer drill assembly of claim 9,wherein the compact cutting inserts comprise carbide buttons.
 12. Theunderreamer drill assembly of claim 1, wherein the center shaft, bitbody, and underreamer arms are modular.
 13. The underreamer drillassembly of claim 12, wherein the bit body is adapted to be removed andreplaced with a second bit body having a different diameter.
 14. Theunderreamer drill assembly of claim 12, wherein the center shaft isadapted to be removed and replaced with a second center shaft that isadapted to couple to a different drive mechanism.
 15. The underreamerdrill assembly of claim 1, wherein the center shaft comprises a driveadapter.
 16. The underreamer drill assembly of claim 15, wherein thedrive adapter is configured to receive rotational torque, impact energy,vibration, or linear feed force from a drive mechanism.
 17. Theunderreamer drill assembly of claim 16, wherein the adapter isconfigured to move the center shaft in response to the rotationaltorque, impact energy, vibration, or linear feed force.
 18. Theunderreamer drill assembly of claim 16, wherein the drive mechanism is adown hole hammer or a drill rod for hydraulic hammer.
 19. Theunderreamer drill assembly of claim 1, wherein the center shaft isadapted at or near the distal end to couple to the bit body via a hex orspline drive mechanism.
 20. An underreamer drill assembly comprising: acenter shaft having a central axis, a proximal end, and a distal end,wherein the center shaft is configured to receive drive energy from adriver at the proximal end and adapted to couple to a bit body via a hexdrive mechanism at the distal end; a bit body adapted to couple to thecenter shaft via the hex drive mechanism and configured to couple to atleast one underreamer arm; and at least one underreamer arm configuredto underream a bore hole having a side wall; wherein the center shaft,bit body, and underreamer arm form a distal cutting face that issubstantially perpendicular to the central axis; wherein the underreamerdrill assembly is configured to direct air, water, or flush mediasubstantially along the central axis and perpendicular to the distalcutting face; and wherein the center shaft is configured to transmitdrive energy from the driver to the at least one arm and move the atleast one arm from a retracted position to an extended position.